13) Alkenes

Question 1

Where are the 4 electrons of each carbon of the double bond in an alkene?

Answer 1

3 are used in 3 sigma bonds

1 is in a p-orbital

Question 2

Describe a pi bond (3 points)

Answer 2

formed by the sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond
each carbon atom contributes 1 e- to the e- pair in the pi bond
pi-electron density is concentrated about and below the plane

Question 3

What effect does the pi-bond have on the two carbon atoms of a double bond?

Answer 3

locks the carbon atoms in position and prevents them from rotating around the double bond

Question 4

What is the shape and bond angle of carbon atoms of the double bond?

Answer 4

trigonal planar shape

120 degrees

Question 5

Define stereoisomers

Answer 5

compounds with the same structural formula but with a different arrangement of atoms in space

Question 6

Define E/Z isomerism

Answer 6

a type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the double bond

Question 7

Define cis-trans isomerism

Answer 7

a special type of E/Z isomerism in which there are two non-hydrogen groups and two hydrogen atoms around the C=C double bond

Question 8

What saying can you use to remember E/Z and cis-trans isomerism?

Answer 8

ET phone home

Question 9

In Cahn-Ingold-Prelog nomenclature, what 2 things should you remember when assigning priority?

Answer 9

the higher the atomic number, the higher the priority

if two atoms attached to a carbon atom in the double bond are the same, use the first point of difference

Question 10

Why are alkenes more reactive than alkanes?

Answer 10

pi bond

being on the outside of the double bond pi-electrons are more exposed than e- in the sigma bond

Question 11

alkene -> alkane

Answer 11

+ H2
Ni catalyst at 423K
addition / hydrogenation

Question 12

alkene -> haloalkane

Answer 12

+Cl2/Br2/hydrogen halide

addition/halogenation

Question 13

What should you remember when an unsymmetrical alkene reacts with an unsymmetrical compound?

Answer 13

two products are possible

Question 14

How can you test for unsaturation?

Answer 14

bromine water

turns from orange to colourless

Question 15

akene -> alcohol

Answer 15

+H20(g)
H3PO4
addition/hydration

Question 16

Name the mechanism used for the addition reactions of alkenes

Answer 16

electrophilic addition

Question 17

Define electrophile

Answer 17

an atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond

Question 18

In an electrophilic addition reaction of a hydrogen halide with an alkene, how does the bond break and where are the 3 curly arrows?

Answer 18

by heterolytic fission
from the C=C bond to the partially positive H of the hydrogen halide
from the H-halide bond to the partially negative halide atom
from an e- pair of the negative halide ion to the positive carbon of the carbocation

Question 19

State Markownikoff’s rule

Answer 19

addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the most stable carbocation

Question 20

What does a carbocation contain?

Answer 20

a positively charged carbon atom

Question 21

Explain why a tertiary carbocation is the most stable in comparison to a primary or secondary carbocation

Answer 21

each akyl group donate and pushes e- towards the positive charge of the carbocation
the positive charge is spread over the alkyl group
the more alkyl groups, the more charge is spread out - therefore, the carbocation is more stable

Question 22

Define addition polymerisation

Answer 22

formation of a very large molecular chain, by repeated addition reactions of many monomers (unsaturated alkene molecules)

Question 23

Give a key feature of an addition polymer

Answer 23

high molecular mass

Question 24

How are synthetic polymers usually named?

Answer 24

after the monomer that reacts to form their giant molecules, prefixed by ‘poly’

Question 25

Define repeat unit

Answer 25

the specific arrangement of atoms in the polymer molecule that is repeated

Question 26

What 2 conditions are needed for polymerisation?

Answer 26

high temperature

high pressure

Question 27

Why are alkene-based polymers of environmental concern?

Answer 27

their lack of reactivity makes many non-biodegradable

Question 28

Explain how polymers are recycled and the advantages and disadvantages of this are

Answer 28

polymers are sorted by type, chopped into pellets, washed, dried and melted. Then cut into pellets to be made into new products

advantages: conserves finite fossil fuels; reduces landfill
disadvantages: doesn’t work for mixed polymers

Question 29

Why is PVC recycling hazardous?

Answer 29

high chlorine content and additives

Question 30

Describe the effect of incineration of polymers

Answer 30

produces heat, generating steam to drive a turbine producing electricity

Question 31

Describe feedstock recycling and its advantages

Answer 31

chemical/thermal processes to reclaim monomers, gases, or oil from waste polymers which can then be used as raw materials for the production of new polymers
advantages: able to handle unsorted/unwashed polymers

Question 32

What are biodegradable polymers usually made from?

Answer 32

starch or cellulose

Question 33

How are biodegradable polymers broken down?

Answer 33

by microorganisms into water, carbon dioxide and biological compounds

Question 34

What is special about photodegradable polymers?

Answer 34

contain bonds that are weakened by absorbing light to start degradation - or light-absorbing additives

Question 35

Suggest why a polymer with an -OH group present can dissolve in water

Answer 35

can form hydrogen bonds with water

Question 36

The combustion of waste polymers can be used for energy production. What problem is caused by disposing of PVC in this way?

Answer 36

hydrogen chloride gas is produced